In an automobile coating line requiring coating at high quality, a rotary atomizing electrostatic coating machine has been popularized since it is excellent in atomizing a coating material and has high deposition efficiency.
FIG. 2 shows such an existent rotary atomizing electrostatic coating machine 21 in which a rotary atomizing head 24 is fitted at the top end of a tubular rotary shaft 23 of a built-in air motor 22.
In the rotary atomizing head 24, a coating material chamber 28 for receiving supply of a coating material from a fine tubular nozzle 27 inserted through the tubular rotary shaft 23 is formed on the rear side of an inner bell 26 fitted to the frontal side of a bell body 25, and a substantially frustconical rim 30 is formed on the frontal side thereof for spreading the coating material jetted out centrifugally from a coating material jetting port 29 formed to the peripheral surface of the coating material chamber 28 and atomizing the same at the top end thereof.
Patent Document 1: Japanese Patent Laid-Open No. 9-94489
Patent Document 2: Japanese Patent Application No. 2003-374909
According to them, when a coating material is supplied from the fine tubular nozzle 27 while rotationally driving the rotary atomizing head 24 at a high speed by the air motor 22, the coating material is caused to flow into the coating material chamber 28, hit against the rear side of the inner bell 26, blown out to the peripheral surface of the coating material chamber 28, caused to flow from the coating material discharge hole 29 to a rim 30 and atomized at a top end thereof.
By the way, abatement of the VOC (Volatile Organic Compound) and CO2 has been highly publicized with an environmental point of view and it has been demanded also in the field of the coating industry to suppress the VOC to be discarded by improving the deposition efficiency of the coating material thereby eliminating the wasteful loss of the coating material and, at the same time, suppressing occurrence of CO2 by decreasing the energy consumed in the combustion treatment of volatile organic substances to be discarded, etc.
In this case, for enhancing the deposition efficiency, atomization of the coating material is necessary and it may be considered to supply air at a high pressure to the air motor 22 of the coating machine 21 to increase the rotational speed of the rotary atomizing head 24 for atomization. However, increase of the rotational speed not only increases the generation amount of scattering coating mist but also increases the consumption amount of the energy, and this is contrary to the demand for the abatement of the VOC and CO2.
In a case where it is possible to attain the atomization of the coating material by low speed rotation of the rotary atomizing head 24, the deposition efficiency will be improved and abatement of the VOC and CO2 can also be attained.
According to the experiment made by the inventor, it has been found that atomization is possible even at low speed rotation when the amount of the coating material to be supplied from the fine tubular nozzle 27 to the coating material chamber 29 is decreased. However, no necessary jetting amount can be obtained in this case to result in a problem that a coating film of a necessary thickness can not be formed within a restricted coating time to a work such as car bodies conveyed continuously on a conveyor thereby lowering the production efficiency.